1. Field of the Invention
The present invention relates to a semiconductor memory device and manufacturing method thereof, and particularly, to a non-volatile semiconductor memory device using an ONO (Oxide-Nitride-Oxide) film as an electric charge holding layer and a manufacturing method thereof.
2. Description of the Background Art
Conventionally, among non-volatile semiconductor memory devices, for example an SONOS (Silicon-Oxide-Nitride-Oxide-Semiconductor) type non-volatile semiconductor device has been known as a structure particularly suitable for miniaturization.
In the following, the procedure of forming a cell array in a conventional SONOS type semiconductor memory device is described.
First, on a silicon substrate, a three-layer structured film of an oxide film, a nitride film, and an oxide film (an ONO film) is formed by CVD (Chemical Vapor Deposition) method or the like. This ONO film serves as a charge holding layer in the cell.
Next, after removing the ONO film using a resist pattern which has an opening in a diffusion layer interconnection region portion of the cell array, for example arsenic is injected (as for ion concentration, for example about 3.0×1015 (1/cm2)) to form an n+ diffusion layer region.
Then, by removing the resist and oxidizing the n+ diffusion layer region, an element isolating region is formed thereon. Here, regions except for the diffusion layer region are not oxidized by the effect of the remaining nitride film of the ONO film.
Further, a polysilicon or the like to be a gate electrode is deposited by CVD method or the like. Then, a word line pattern is formed by a resist, and the polysilicon is removed from the regions except for the gate electrode.
Through the process described above, the cell of the SONOS type semiconductor memory device is formed.
The manufacturing method of the semiconductor memory device as described above, however, has been associated with the following problem.
In the procedure of the cell array portion formation described above, the ONO film is removed with a prescribed pattern, to which the n+ diffusion layer region is formed. On the diffusion layer region surface, a region formed by LOCOS (Local Oxidation of Silicon) method as an element isolating region is formed. Thus, ONO films among each element are isolated.
Here, in the step of forming the region formed by LOCOS method (the LOCOS region), sometimes the diffusion layer (such as arsenic) diffuses in the gate length direction by a thermal oxidation process, whereby the effective gate length of the cell becomes short. This hinders miniaturization (scaling) of the cell size.
On the other hand, a SONOS type semiconductor memory device has been produced, which is manufactured without forming LOCOS region and etching the ONO film for isolation, considering the effect to the gate length.
In this case, however, the ONO film will be used as a through film for transmitting the injected diffusion layer (arsenic), whereby the electric reliability of the ONO film (the charge holding layer) is degraded. Further, since the ONO film is not isolated among each element, when the cell size is reduced, charges may diffuse and thus affect adjacent cells.
Japanese Patent Laying-Open No. 2001-351993 (Conventional Example 1) discloses a semiconductor memory device in which an HDP oxide film is embedded in concave portions of a silicon substrate and polished through CMP method, so that the HDP oxide film is left only on an impurity diffusion region.
Japanese Patent Laying-Open No. 2000-091450 (Conventional Example 2) discloses a non-volatile semiconductor memory device in which a titanium-silicide film is formed on the surfaces of source, a drain diffusion layer and a control gate electrode in order to reduce resistance.
Japanese Patent Laying-Open No. 10-012750 (Conventional Example 3) discloses a non-volatile semiconductor memory device in which a floating gate electrode serving as a charge holding portion of a memory cell is formed by two layers of polycrystalline silicon films electrically connected to each other. A silicon oxide film is formed between the two layers of polycrystalline silicon films and a word line (a polycrystalline silicon film).
Conventional Example 1, however, discloses a non-volatile memory including a floating gate electrode and a control gate electrode, of which basis is totally different from the present invention having an ONO film as a charge holding layer.
As for Conventional Example 2, it does not disclose a structure in which a gate electrode is arranged perpendicular to a diffusion layer to which a silicide film is formed, and therefore the semiconductor memory device according to Conventional Example 2 and the semiconductor memory device according to the present invention are totally different in the structure.
As for Conventional Example 3, two layers of polycrystalline silicon films serve as charge holding layers, and therefore it is different from the present invention in which an ONO film is used as the charge holding layer.